Taipei, May 26 (CNA) The source of a cluster infection of the new coronavirus disease (COVID-19) on board a Taiwanese Navy ship in March originated in Taiwan rather than Palau, the Central Epidemic Command Center (CECC) said Tuesday.

CECC advisor Chang Shan-chwen said the earliest known confirmed case of the disease on board the Panshi fast combat support ship got sick on March 10, which was before the ship visited Taiwan's diplomatic ally Palau on a training mission from March 12-15.

That meant the infection was likely brought aboard the ship before it left Taiwan on March 5, Chang said, though the CECC's investigation did not find the original source of the virus and it has decided to close the case.

The crew of the Panshi and the two other ships in the flotilla visiting Palau actually boarded on Feb. 21 and were required to stay on the ships until they left, but it was later revealed that they were allowed to leave the ship for the Feb. 28 holiday, leading to speculation that the crew could have picked up the virus then.

The ships then returned to their naval base in Zuoying in the Kaohsiung area on April 9, but those on board were not allowed to disembark until April 14 and 15 in accordance with quarantine requirements.

The cluster infections on the Panshi were reported by the CECC starting on April 18, leading to the 744 people who participated in the mission being collectively quarantined that day to prevent community spread of the virus.

CECC chief Chen Shih-chung initially speculated on April 18 that the infections may have originated in Palau, only to backtrack a day later.

Chang said the CECC's investigation into the cluster infection found four waves of infection on the ship.

The first originated in Taiwan and led to the first case on March 10, the second came around March 20, the third occurred in early April and the fourth came in mid-April.

A total of 36 Panshi crew members were eventually confirmed as having COVID-19.

The fact that crew were already infected by March 10 meant they could have brought the virus to Palau in mid-March.

On April 20, however, Palau Health Minister Emais Roberts released a statement saying no one in his country has tested positive or shown any symptoms of the disease.

So what happened?

Chang suggested that only one crew member was infected at the time of the stop in Palau visit because the next case did not show symptoms until March 15, and that while the initial crew member with the disease got off the ship in Palau, he came mostly in contact with others from the ship rather than Palau residents.

Meanwhile, the CECC also said a common symptom of those on the Panshi who were infected was a loss of the sense of smell, a symptom more associated with cases seen in Europe and the United States and not with the COVID-19 cases seen in Taiwan up to that time.

The CECC found, however, that the strain did not match strains from Europe but rather a "Southeast Asian" strain found in Philippines, according to Chang, but he did not comment on how prevalent that strain has been in Taiwan.

To date, 32 crew members have recovered and four are still being treated in the hospital, said Chen, the convener of the CECC's advisory specialist panel.

Taiwan has recorded 441 cases of the disease, with seven deaths, according to CECC statistics.